1. Field of the Invention
This invention relates to a display, especially to a pre-charging apparatus and a pre-charging apparatus operating method applied in a source driver circuit of a display.
2. Description of the Related Art
With the increasing size of the LCD display panel, the load driven by the output operational amplifier (OP) disposed in the source driver circuit of the LCD display becomes heavier, and the output operational amplifier will encounter problems with power consumption and over-heating. Therefore, large-size LCD display panels may be required to reduce power consumption and prevent over-heating.
In order to reduce power consumption and prevent over-heating, a pre-charging function is used in some source driver circuits of the LCD display. Before the output operational amplifier in the source driver circuit starts to drive the output voltage of the source driver circuit, an external operational amplifier is used to perform the pre-charging function on the output voltage of the source driver circuit to make the output voltage level of the source driver circuit closes to a default output voltage, so that the voltage range necessary to be driven by the output operational amplifier of the source driver circuit is decreased to save power and prevent over-heating.
Using a 8-bit data signal as an example, the source driver circuit will capture the most significant bit (MSB) of a bit of a current data and the most significant bit of a bit of a former data and compare them. If the most significant bit is converted from 0 to 1 or from 1 to 0, the source driver circuit will start the pre-charging function to control an external pre-charging unit to increase the output voltage level of the source driver circuit to ¾ and ¼ of the default output voltage.
However, as shown in FIG. 1, 256 grey-levels L0˜L255 can be divided into a red region RR including grey-levels L0˜L127 and a blue region BR including grey-levels L128˜L255. When the data signal is transited from the red region RR to the blue region BR or transited from the blue region BR to the red region RR, the source driver circuit will start the pre-charging function. When the large-size LCD panel displays an image having the grey-levels L0˜L255, the grey-level inversion will occur near a junction of the red region RR and the blue region BR (e.g., the grey-levels L127˜L129), and abnormal horizontal bright lines or dark lines will be shown in the image displayed on the large-size LCD panel; therefore, the image displaying quality of the large-size LCD panel will be poorer.
Because the most significant bit of the data signal is used to determine whether the pre-charging should be performed, as shown in FIG. 2, during the transition of the data signal DAT from the grey-level L127 to the grey-level L129, the voltage difference between the grey-levels L127 and L129 may be smaller than 5 mV, and when the data signal DAT is transited from the grey-level L127 to the grey-level L128 at first, the data signal DAT is transited from the red region RR to the blue region BR; therefore, the source driver circuit will start the pre-charging function. Then, when the data signal DAT is transited from the grey-level L128 to the grey-level L129, the output operational amplifier in the source driver circuit will drive the output voltage of the source driver circuit. Therefore, under the operation of high-frequency (240 Hz) frame rate, when the large-size LCD panel displays the grey-level L128 of the image, the output voltage is not totally driven by the output operational amplifier in the source driver circuit, the abnormal horizontal bright lines (higher than the grey-level L129) or dark lines (lower than the grey-level L128) would be easily shown in the image displayed on the large-size LCD panel. Although these abnormal horizontal bright lines or dark lines can be slightly improved by adjusting pre-charging voltage or lowering the frame rate to 120 Hz, it is not suitable for production because the voltage difference between the grey-levels L127 and L129 is too small to be well-adjusted.
Therefore, embodiments of the invention provide a pre-charging apparatus and a pre-charging apparatus operating method applied in a source driver circuit of a display to solve the above-mentioned problems.